1. Field of the Invention
In the communication system for time-sharing multiplex transmission of digital signals by the use of optical cables, this invention relates to a method for the synchronization of system timing required in the apportionment of time among packets of signals transmitted from the personal stations of the system in the optical cables and to a device for working this method.
2. Description of the Prior Art
In recent years, the development of a local area network (LAN) for transporting signals in the form of packets by the use of routes formed by laying coaxial cables in the form of buses has culminated.
This LAN which makes use of packets requires installation, as in laboratories, of transmission cables adapted to effect transmission in both directions and connection to these transmission cables of numerous personal stations. From these personal stations, messages divided into data blocks each of 1000 to 2000 bits, for example, are transmitted through the transmission cables. The individual messages are each prefixed with a header covering such information as address and serial number.
In the LAN of this nature, the network itself is a passive transmission medium totally devoid of any control function and the individual personal stations have such control functions thoroughly distributed among themselves.
At a given personal station, therefore, transmission of a message is started when an idle channel is available in the cables. When a packet of message transmitted from one personal station collides with a packet of message transmitted from another personal station, these two personal stations discontinue the transmission of their messages. The personal station which has discontinued the transmission, on elapse of a random queuing time, tries to resume the transmission of the message. This system is called "Carrier-Sense Multiple Access/Collision-Detection(CSMA/CD)"
Incidentally in this LAN, since individual personal stations have liberty of starting data transmission at any time desired, there is a possibility of packets colliding with each other on the transmission line. The LAN, therefore, entails a problem that it has no fixed transmission delay time. Because of this problem, it does not perfectly fit the real-time transmission such as for the conversation type vocal communication in which real-time transmission and reception are highly valued.
The problem is solved by keeping a master station in operation permanently and enabling the individual personal stations to reserve right to channel access with the master station. In this setup, however, a failure in the master station could result in total inactivation of data communication. This possibility impairs the reliability of this system.
For the solution of the problem just mentioned, there has been proposed the modified Ethernet system which is capable of providing real-time transmission without depriving the personal stations of their mutual equality.
In accordance with this signal transmission system, the frames which are periodically repeated along time axis are each divided into a plurality of blocks along the time axis. With these blocks as the unit, this system provides the personal stations with chances for packet type communication.
In this signal transmission system, all the personal stations are equally entitled to the use of idle blocks. In case where a given personal station occupies a specific block over a duration necessary for signal transmission, that personal station is periodically given a chance for signal transmission in each of frames repeated on the time base. Thus, this system permits the personal stations to effect real-time transmission of signals by making use of the function described above.
One typical frame configuration for the signals to be used in the aforementioned digital signal transmission system is illustrated in FIG. 1.
Each of the frames which are repeated periodically on the time base consists of N blocks (#1 through #N). And each of the blocks consists of various bit rows, b.sub.1 through b.sub.9, as shown below.
b.sub.1 . . . Rear guard time PA1 b.sub.2 . . . Preamble PA1 b.sub.3 . . . Address field PA1 b.sub.4 . . . Distance code field PA1 b.sub.5 . . . Control field PA1 b.sub.6 . . . Data field PA1 b.sub.7 . . . Frame check sequence (FCS) field PA1 b.sub.8 . . . End flag PA1 b.sub.9 Front guard time
The bit rows b.sub.2 through b.sub.5 and the bit rows b.sub.7 and b.sub.8 are essential components for a packet. These bit rows are collectively referred to as "overhead bits." The two bit rows, b.sub.1 and b.sub.9, are collectively referred to as the "guard time."
The term "guard time" means "empty bit rows" which are intended to preclude the situation in which packets in adjacent blocks may possibly be caused to overlap, if partially, owing to the delay time which occurs during the propagation of signals on a coaxial cable.
In the bit rows forming this guard time, the backward guard time b.sub.1 serves to protect the trailing one of any two adjacent packets against the trouble of overlapping and the forward guard time b.sub.9 similarly to protect the leading packet against the trouble.
The sum of the number of bits of the rear guard time b.sub.1, and that of bits of the front guard time b.sub.9, will be represented as g bits and the guard time (b.sub.1 +b.sub.9) will be represented hereinafter as .tau.(g).
In the digital signal transmission system proposed as described above, when none of the personal stations in the system is transmitting signal, all the personal stations have a chance, equally and at any time at all, to start sending out signals in the aforementioned frame configuration. Thus, the particular personal station which is the first to start sending out signal onto the transmission cable will take the initiative in the synchronization of frames.
Once the frame synchronization has been established as described above, all the personal stations are enabled to keep watch on the state of signals being transmitted on the transmission cable.
As will be described fully afterward, the user devices at the personal stations are each provided with a memory adapted to memorize the condition of occupation of individual blocks by signals in the frames. Thus, all the personal stations are allowed to register relevant blocks based on the incoming packet signals addressed to themselves.
After the particular personal station has established the frame synchronization, any of the other personal stations is allowed to send out packet signals by selecting empty blocks based on the information stored in the aforementioned memory and loading these empty blocks with packet signals desired to be transmitted.
In this case, the timing by which the personal stations are allowed to send out their own packet signals poses a problem.
For the sake of explanation, let us assume that, as illustrated in FIG. 2, a coaxial cable 3 has its opposite ends connected to impedance matching terminators 1 and 2, a personal station C is located at the middle point of the coaxial cable 3, and a personal station S located between the personal station C and the terminator 1 is already in the process of transmitting signals on the coaxial cable 3.
In this case, the packet signals which are being sent out by the personal station S are received by the personal station C and the other personal stations, R.sub.1 through R.sub.4, on the coaxial cable 3 at different points of time, depending on the variation in the signal propagation delay time on the cable 3.
If the personal stations randomly send out their own signals without paying any respect to the other personal stations, then there is a fair possibility that the packets issuing from such personal stations will overlap (collide with) each other on the coaxial cable 3.
For the purpose of precluding this detestable phenomenon, the aforementioned signal transmission system makes effective use of the aforementioned concept of guard time .tau.(g), in establishing the synchronization of system timing.
To be more specific, in this signal transmission system, the guard time .tau.(g), is fixed at two or more times of the signal propagation delay time required to cover the distance between the centrally located personal station C, datum position, and the most distant personal station and the transmission of signals is effected so that, at the receiving point of the centrally located personal station C, the packets issuing from the individual personal stations will be arranged as separated by equal intervals.
FIG. 3 provides more specific illustration of the working of the signal transmission system. The diagram depicts the system on the assumption that while the personal stations of the system are connected as illustrated in FIG. 2, the personal station S is already in the process of transmitting signals and the other personal stations, R.sub.1 through R.sub.4, are about to start sending out packet signals.
In this case, the personal stations R.sub.1 through R.sub.4 which follow the personal station S in the order of signal transmission determine their own points of timing for sending out their own transmission packets so that the personal station C, the datum points, will begin to receive the transmitted packets one guard time .tau.(g), after the personal station C completes reception of the transmission packets (or transmission S packets) from the personal station S.
To determine such timing for the issuance of signals, the personal stations R.sub.1 through R.sub.4, on receiving the packet signals transmitted on the coaxial cable 3, first examine the address field (b.sub.3) of the received packet signals and discern the reception of packets from the personal station S (reception S packets).
Further, the personal stations R.sub.1 through R.sub.4, based on the signal propagation delay times between the personal station S, the centrally located personal station C, and their own stations, determine the points of time at which the arrival of reception S packets at the point of reception of the personal station C is completed.
These points of time, as illustrated in FIG. 3, are later than the points of time at which the reception of the reception S packets at the personal stations R.sub.1 and R.sub.2 is completed and earlier than the point of time at which the reception of the reception S packets at the personal stations R.sub.3 and R.sub.4 is completed.
After the personal stations R.sub.1 through R.sub.4 have determined the point of time at which the reception of reception S packets is completed with reference to the personal station C as the datum point, the particular one of these four personal stations which desired to send out signals begins to send out packet signals [or transmission R.sub.i packets (i=1 to 4)] at the point of time which is earlier than the determined point of time mentioned above by an interval equivalent to the signal propagation delay time required to cover the distance between its own station and the personal station C.
The packet signals which have been sent out as described above begin to be received (as reception R.sub.i packets) at the personal station C as the datum point after elapse of one guard time .tau.(g), from the time of completion of the reception of reception S packets as illustrated in FIG. 3.
This adjustment of the timing for sending out signals is accomplished by establishing frame synchronization and block synchronization at all the personal stations concerned.
Specifically, the personal stations are adapted, as described fully afterward, to reset periodically at a fixed timing the frame counter and the block counter which take count of the clock signals fed out by their own oscillators. Because of this function, the personal stations are able to establish system timing of frame synchronization and block synchronization, etc. within the tolerance of frequency of the clock signals.
At the personal station R.sub.3 located on the outer side of the personal station C as viewed from the personal station S, for example, the aforementioned frame counter and block counter are reset on elapse of the time, .tau.(b.sub.9), calculated by the following formula (hereinafter referred to as "front guard time"): EQU .tau.(b.sub.9)=.tau.(g)/2-.tau.(CR.sub.3) (1)
from the time of the rear end of the reception S packets, and these counters are controlled so as to start the second block, #2, at that time. In the formula, .tau.(CR.sub.3) denotes the time for propagation of signals between the personal station C and the personal station R.sub.3.
At the personal station R.sub.2 which is located between the personal station S and the personal station C, the front guard time, .tau.(b.sub.9), is found as follows: EQU .tau.(b.sub.9)=.tau.(g)/2+.tau.(CR.sub.2) (2)
In this formula, .tau.(CR.sub.2), denotes the time for propagation of signals between the personal station C and the personal station R.sub.2.
Then at the personal station R.sub.1 which is located on the outer side of the personal station S as viewed from the personal station C, the front guard time, .tau.(b.sub.9), is found as follows. EQU .tau.(b.sub.9)=.tau.(g)/2+.tau.(SC)-.tau.(SR.sub.1) (3)
In the formula, .tau.(SC) denotes the time for propagation of signals between the personal station S and the personal station C and .tau.(SR.sub.1) the time similarly between the personal station S and the personal station R.sub.1.
The personal stations R.sub.1 through R.sub.4 start transmitting packets of signals on elapse of the respective intervals, t.sub.1 through t.sub.4, which are determined by the following formulas, from the time of reset of said counters: ##EQU1## In other words, the rear guard times, .tau.(b.sub.1), for the signals transmitted from the personal stations R.sub.1 through R.sub.4 equal the intervals indicated by the foregoing formula (4).
In the LAN, the code distance indicative of the position of the particular station that holds the priority (or leadership) in frame synchronization is placed, as illustrated in the packet construction of FIG. 1, in the distance code field (b.sub.4) of the master packet issuing from that (master) station. In the distance code field (b.sub.4) in the packet issuing from any personal station other than the master station, a specific code different from the distance code assigned to that personal station is placed. This code is intended to designate that the station is not a master station.
Each of the stations other than the master station keeps watch on the distance code field (b.sub.4) in every packet it receives. When it detects the distance code field (b.sub.4) indicative of the distance code (the distance code field of the master packet), the distance code field (b.sub.4) is transfered to the receive logical circuit.
The receive logical circuit incorporates an arithmetic circuit, which determines the front guard time, .tau.(b.sub.9), in accordance with the pertinent one of the aforementioned three formulas (1) through (3), using the distance code of the master packet so transferred and the distance code of the particular personal station that has received the distance code. The signal resulting from this arithmetic operation is fed to a programmable timer, for example, and the timer is operated from the rear end of the master packet. By resetting the aforementioned counters with the output of this timer, therefore, the frame synchronism and the block synchronism of the station can be established.
By the term "programmable timer" as used herein is meant a general-purpose circuit so adapted that, when started by the supply of a signal corresponding to the front guard time, .tau.(b.sub.9), designates with a rise or fall of signal the time which arrives after elapse of the front guard time, .tau.(b.sub.9), from the time the timer is started.
Unfortunately, the LAN constructed as described above entails the following drawbacks.
(1) Since this system uses coaxial cables, it is susceptible of the adverse effects such as of the electromagnetic induction noise. Thus, the system is not suitable for installation within a plant which is using large electric power.
(2) The highest possible rate of signal transmission obtained by the coaxial cables is only about 50 Mbps. Thus, the system is not suitable for data transmission which is required to be effected at a higher rate.
(3) The system necessitates arithmetic operations to be performed on the pertinent one of the three formulas (1) through (3) as applied to the positional relations of the master station and the individual personal stations. For the purpose of such arithmetic operations, the system requires a complicated circuit.